This invention is directed to optical devices and related methods. More particularly, the invention provides a method of manufacture and a device for emitting electromagnetic radiation using semipolar or non-polar gallium containing substrates such as GaN, MN, InN, InGaN, AlGaN, and AlInGaN, and others. Merely by way of example, the invention can be applied to optical devices, lasers, light emitting diodes, solar cells, photoelectrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, among other devices.
In the late 1800's, Thomas Edison invented the light bulb. The conventional light bulb, commonly called the “Edison bulb,” has been used for over one hundred years for a variety of applications including lighting and displays. The conventional light bulb uses a tungsten filament enclosed in a glass bulb sealed in a base, which is screwed into a socket. The socket is coupled to an AC or DC power source. The conventional light bulb can be found commonly in houses, buildings, and outdoor lightings, and other areas requiring light or displays.
Unfortunately, drawbacks exist with the conventional Edison light bulb. First, the conventional light bulb dissipates much thermal energy. More than 90% of the energy used for the conventional light bulb dissipates as thermal energy. Second, reliability is less than desired—the conventional light bulb routinely fails due to thermal expansion and contraction of the filament element. In addition, conventional light bulbs emit light over a broad spectrum, much of which does not result in illumination at wavelengths of spectral sensitivity to the human eye. Finally, conventional light bulbs emit light in all directions. They therefore are not ideal for applications requiring strong directionality or focus, such as projection displays, optical data storage, or specialized directed lighting.
In 1960, the laser was first demonstrated by Theodore H. Maiman at Hughes Research Laboratories in Malibu. This laser utilized a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nm. By 1964, blue and green laser output was demonstrated by William Bridges at Hughes Aircraft utilizing a gas Argon ion laser. The Ar-ion laser utilized a noble gas as the active medium and produced laser light output in the UV, blue, and green wavelengths including 351 nm, 454.6 nm, 457.9 nm, 465.8 nm, 476.5 nm, 488.0 nm, 496.5 nm, 501.7 nm, 514.5 nm, and 528.7 nm. The Ar-ion laser had the benefit of producing highly directional and focusable light with a narrow spectral output, but the wall plug efficiency was less than 0.1%. The size, weight, and cost of the laser was undesirable as well.
As laser technology evolved, more efficient lamp pumped solid state laser designs were developed for the red and infrared wavelengths, but these technologies remained a challenge for blue and green lasers. As a result, lamp pumped solid state lasers were developed in the infrared, and the output wavelength was converted to the visible using specialty crystals with nonlinear optical properties. A green lamp pumped solid state laser had 3 stages: electricity powers lamp, lamp excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The resulting green and blue lasers were called “lamped pumped solid state lasers with second harmonic generation” (LPSS with SHG). These had wall plug efficiency of ˜1%, and were more efficient than Ar-ion gas lasers, but were still too inefficient, large, expensive, and fragile for broad deployment outside of specialty scientific and medical applications. Additionally, the gain crystal used in the solid state lasers typically had energy storage properties which made the lasers difficult to modulate at high speeds, limiting broader deployment.
To improve the efficiency of these visible lasers, high power diode (or semiconductor) lasers were utilized. These “diode pumped solid state lasers with SHG” (DPSS with SHG) had 3 stages: electricity powers 808 nm diode laser, 808 nm excites gain crystal which lases at 1064 nm, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm. The DPSS laser technology extended the life and improved the wall plug efficiency of the LPSS lasers to 5-10%. This sparked further commercialization into specialty industrial, medical, and scientific applications. The change to diode pumping, however, increased the system cost and required precise temperature controls, leaving the laser with substantial size and power consumption. The result did not address the energy storage properties which made the lasers difficult to modulate at high speeds.
As high power laser diodes evolved and new specialty SHG crystals were developed, it became possible to directly convert the output of the infrared diode laser to produce blue and green laser light output. These “directly doubled diode lasers” or SHG diode lasers had 2 stages: electricity powers 1064 nm semiconductor laser, 1064 nm goes into frequency conversion crystal which converts to visible 532 nm green light. These lasers designs are intended to provide improved efficiency, cost and size compared to DPSS-SHG lasers, but the specialty diodes and crystals required make this challenging today. Additionally, while the diode-SHG lasers have the benefit of being directly modulated, they suffer from sensitivity to temperature which limits their application. Further, the spectral linewidth of SHG-based lasers is typically very narrow at about 0.1 nm, which can lead to a severe image distortion called speckle in display applications.